In relation to a needle-free syringe (needleless syringe) with which the injection is performed without using any injection needle, a construction is adopted in some cases such that an injection component is injected or allowed to inject by applying a pressure to an accommodating chamber in which an injection solution is accommodated, by means of a pressurized gas or a spring. However, in the case of the needle-free syringe having the conventionally known construction, the reproducibility is unsatisfactory in relation to the depth and the injection amount of the injection solution. Therefore, it is difficult to affirm that such a needle-free syringe generally comes into widespread use.
Accordingly, such a technique is disclosed that a propellant charge, which is composed of a mixture of two types of powders, i.e., a high speed combustion powder and a low speed combustion powder, is utilized to adjust the output pressure (injection pressure) for the injection solution in a plurality of levels or stages (see, for example, Patent Document 1). Specifically, the injection solution is firstly allowed to inject by applying a large force to a piston by the combustion of the high speed combustion powder. As a result, the injection solution penetrates through a skin of a human body or the like, and the injection solution is fed into the body. After that, a pressure is continuously applied to such an extent that the injection solution can be diffused in the skin by the combustion of the low speed combustion powder.
Patent Document 2 discloses such a technique that an injection solution is administered in two stages by using a needle-free syringe. In this technique, the injection solution is allowed to inject by applying a high pressure thereto so that the injection solution penetrates into the skin, and then the pressure, which is applied to the injection solution, is lowered so that it is contemplated to disperse the injection solution in the skin. Further, Patent Document 3 discloses such a technique that the injection pressure for an injection solution is adjusted by the intensity of the electric current by using a magnet and a coil. In this technique, the injection pressure is adjusted so that a high pressure is firstly applied in order that the injection solution penetrates through the skin, and then an approximately constant pressure is provided in order that the desired injection solution is fed or delivered.
The pressure, which is applied to the injection solution when the needle-free syringe is used, is variously adjusted not only for such a purpose that the injection solution is allowed to arrive at the interior of the skin but also for other purposes other than the above. For example, in Patent Document 4, such a description is found that the increase in the pressure applied to the injection solution is unfavorable after the penetration through the skin in order to mitigate the noise generated when the injection solution is allowed to inject by using a pressurized gas.
In this context, the target, for which the injection is performed by using the needle-free syringe, is the living body such as the human body or the like in many cases. Accordingly, a discussion is provided in relation to the behavior of an injected solution with respect to a gel agent generally used for an experiment and the skin of the living body (see, for example, Non-Patent Document 1). This discussion refers, for example, to a correlation between a depth of a hole formed by the injection and a hole depth having a maximum dispersion width, and a correlation between the Young's modulus of the skin and the hole depth. Further, Non-Patent Document 2 refers to a correlation between the dispersion width of an injection solution in the human skin and the nozzle diameter of a needle-free syringe.